A stator of an inner rotor type electrical motor includes a stator core in which a plurality of teeth are provided so as to project from the inner peripheral surface of a yoke formed into an annular shape toward the magnetic pole surface of a rotor. Around the outer periphery of each of the teeth of the stator core, a coil is wound via an insulator.
As examples of the method for forming the stator core, there are available a method in which annular sheet cores in which the yoke and the teeth have beforehand been formed integrally, which are formed of a magnetic steel sheet, are laminated along the direction of the shaft line of an output shaft, and a method in which straight sheet cores (straight cores) obtained by blanking the core and the teeth in a state of being connected in a straight line form for each segment are laminated, and the end parts thereof are connected to each other so that an annular shape is formed.
In the former method, the stator can be formed by two processes: a blanking process and a laminating process. In this method, since the inside portion of stator is also blanked from the magnetic steel sheet at the same time in the blanking process, many useless portions are produced undesirably. In contrast, the latter method has high production efficiency because such a useless blanking process is little.
For example, in Japanese Patent Application Publication No. H09-308143, to bend the straight core into an annular shape, a V-shaped cutout is formed between the yokes of the segments. By the formation of cutout, deformation stresses relating to the straight core concentrate in the cutout part. Therefore, deformation can be produced by a lower working stress, so that the magnetic circuit performance is also enhanced.
However, the conventional method for forming a straight core type stator has problems described below. The cutout described in Japanese Patent Application Publication No. H09-308143 is designed so that in order for the clearance of cutout to become zero when the straight core is formed into an annular shape, the opening angle of the cutout is (360/α), in which α is the number of teeth. Actually, however, the end faces on the joint side of cutouts come into contact with each other at the time of bending operation, and strain is added thereto. Therefore, an influence is exerted on a magnetic circuit to some degree.
Also, for the straight core described in Japanese Patent Application Publication No. H09-308143, two side wall surfaces of the cutout are caused to abut on each other to increase the dimensional accuracy (the degree of complete round of teeth face) at the time when the straight core is formed into an annular shape. Therefore, the blanking accuracy and the laminating accuracy of straight core must be set high, so that advanced production control is needed.
If the dimensional accuracy is poor, that is, the degree of complete round is low, an influence is exerted on the magnetic circuit of motor, and therefore the energy conversion efficiency is lowered. Besides, noise and the like are produced by vibrations and resonance when the motor is operated.
The present invention has been made to solve the above-described problems, and accordingly an object thereof is to provide an electrical motor in which the dimensional accuracy of a stator can be increased more easily while a process for bending a straight core into an annular shape is made easy, and the influence on a magnetic circuit is kept as little as possible.